Conventionally, couplers have been used for various microwave circuits, such as filter circuits, balanced amplifiers, balanced mixer, and baluns.
FIGS. 6(a) through 6(g) are diagrams showing a coupler that employs conventional ¼-wavelength end short-circuited type coupling lines.
FIG. 6(c) is a top plan view showing a conventional coupler, in which parts that are not seen from the top are indicated by dashed lines. FIG. 6(a) is a longitudinal sectional view of the coupler along line A9–A10 of FIG. 6(c). FIG. 6(b) is a longitudinal sectional view thereof along line A11–A12 of FIG. 6(c). FIG. 6(d) is a transverse sectional view thereof along line A1–A2 of FIG. 6(c). FIG. 6(e) is a transverse sectional view thereof along line A3–A4 of FIG. 6(c). FIG. 6(f) is a transverse sectional view thereof along line A5–A6 of FIG. 6(c). FIG. 6(g) is a transverse sectional view thereof along line A7–A8 of FIG. 6(c).
As shown in FIGS. 6(a) and 6(b), the conventional coupler includes a ground conductor 603 that is formed on an under surface of a first dielectric substrate 601, and a ground conductor 604 that is formed on a top surface of a second dielectric substrate 602.
Further, as shown in FIGS. 6(e) and 6(f), between the first dielectric substrate 601 and the second dielectric substrate 602, there are formed signal input/output line conductors 612 and 613 that employ striplines, and two coupling line conductors 620 and 621 that are adjacent to each other so as to be electromagnetically coupled, in symmetry with respect to the center line of the ground conductor 604.
In addition, via conductors 630, 631, 632 and 633 are filled in through holes that pass through the first dielectric substrate 601 and the second dielectric substrate 602.
As shown in FIGS. 6(a) and 6(b), the via conductors 630, 631 and the via conductors 632, 633 short-circuit not-opposing end portions of the coupling line conductors 620 and 621 to the ground conductors 604 and 603 at a position of line A7–A8 of FIG. 6(c) and at a position of line A1–A2 of FIG. 6(c), respectively, thereby providing inter-digital coupling.
Further, on the side surfaces of the first dielectric substrate 601 and the second dielectric substrate 602, ground conductors 605, 606, 607, and 608 are formed.
As described above, the conventional coupler utilizing the ¼-wavelength end short-circuited type coupling lines is formed using the striplines, with the coupling line conductors 620 and 621 being enclosed with the ground conductors 603, 604, 605, 606, 607, and 608.
The conventional coupler utilizing the ¼-wavelength end short-circuited type coupling lines connects the signal input/output line conductors 612 and 613 to the coupling line conductors 620 and 621 symmetrically with respect to a point in such a manner that the conductors 612 and 613 are not opposing to each other, and an input/output impedance is decided from a distance from the connecting point to the end of the coupling line conductor 620 or 621.
Signal input/output end face electrodes 610 and 611 at the mounting on a printed circuit board are formed on the side surfaces of the first dielectric substrate 601 and the second dielectric substrate 602, and are connected to the signal input/output line conductors 612 and 613, respectively.
Here, the coupling line conductors 620 and 621 each have a length along the length, corresponding to a ¼ wavelength, i.e., a longitudinal length corresponding to ¼ λg (λg is an intra-tube wavelength).
When an analysis is performed on the conventional coupler utilizing the ¼-wavelength end short-circuited type coupling lines, using quasi-TEM approximation based on a known even/odd orthogonal mode excitation method (J. Reed) or using an analyzing method in an even or odd mode, which is disclosed by “Practical Use, Lectures on microwave technology—Theory and Fact—Volume 3, June 2001 (written by Yoshihiro Konishi, published by K-Laboratory)”, in-phase excitation occurs in the even mode while opposite-phase excitation occurs in the odd mode.
In this case, characteristic impedances Zodd and Zeven of coupling transmission lines of the coupling lines in the odd and even modes are represented by [Formula 1] and [Formula 2].
[Formula 1]Zodd=1/(Vp×(C1+2×C12))[Ω][Formula 2]Zeven=1/(Vp×C1)[Ω]
Here, Vp is a speed at which the electromagnetic field propagates through a transmission line. C1 is a capacitance per unit length between the coupling line conductors 620 and 621 (striplines) and the ground conductors 603 and 604, and C12 is a capacitance per unit line between the coupling line conductors 620 and 621.
The degree K of coupling of the conventional coupler that utilizes the ¼-wavelength end short-circuited type coupling lines can be expressed by a following formula, using the characteristic impedances Zodd and Zeven.
[Formula 3]K=20 log {(Zeven−Zodd)/(√2×(Zeven+Zodd))[dB]
By substituting [Formula 1] and [Formula 2] into [Formula 3], following [Formula 4] indicating the coupling degree K is obtained.
[Formula 4]K=20 log {C12/(√2×(C1+C12))}
Thus, the coupling degree K of the conventional coupler that utilizes the ¼-wavelength end short-circuited coupling line is represented as described above.
However, in the above-mentioned conventional coupler utilizing the striplines, it is possible to increase the coupling degree K only by extremely reducing the spacing between two coupling line conductors 620 and 621. But, the minimum spacing between the two coupling line conductors 620 and 621 is limited from the viewpoint of manufacturing.
Recently, a low-temperature co-fired ceramic (LTCC) has been developed, whereby it has become possible to thin an insulating layer and miniaturize the coupler. However, when the insulating layer is thinned, the capacitance C1 per unit length between the coupling line conductors 620 and 621 as the striplines, and the ground conductors 603 and 604 is increased. Accordingly, the coupling degree K of the coupling line is further reduced as expressed by [Formula 4].
To solve this problem, Japanese Patent Application No. Hei. 05-135749 (Japanese Published Patent Application No. Hei. 06-350313) suggests a ¼-wavelength coupling line type directional coupler which is obtained by improving the above-mentioned conventional coupler.
The prior art as disclosed in this publication relates to line conductors mainly using microstrips, but it is easily affected by electromagnetic interference from outside, and further, components cannot be placed above or below the ¼-wavelength coupling line directional coupler, so that it is not suitable for high-density packaging and cannot be miniaturized.
The present invention is made to overcome the above-mentioned conventional problems, and has for its object to provide a coupler having a higher coupling degree K, which is smaller in size and allows higher-density packaging with relative to the prior art.